1. Field of the Invention
The present invention relates to a display device and a driving method thereof, particularly relates to a display device to which an area gray scale method is applied and a driving method thereof.
2. Description of the Related Art
In recent years, a so-called self-light emitting display device of which pixel is formed using a light emitting element such as a light emitting diode (LED) has attracted attention. As a light emitting element used for such a self-light emitting display device, an organic light emitting diode (also referred to as an OLED (Organic Light Emitting Diode), an organic EL element, an electroluminescence (EL) element, or the like) has attracted attention, and has been used for an EL display and the like. Since a light emitting element such as an OLED is a self-light emitting type, it has advantages of high visibility of pixels, no backlight required, high response speed, and the like over a liquid crystal display. Luminance of a light emitting element is controlled by a value of a current flowing through the light emitting element.
As a driving method for controlling a light emitting gray scale of such a display device, there are a digital gray scale method and an analog gray scale method. In the digital gray scale method, a light emitting element is turned on/off by digital control to express a gray scale. On the other hand, the analog gray scale method includes a method for controlling luminance of a light emitting element by an analog manner and a method for controlling a light emitting period of a light emitting element by an analog manner.
In a case of the digital gray scale method, there are only two states: a light emitting state and a non-light emitting state. Therefore, only two gray scales can be expressed if nothing is done. Accordingly, another method is used in combination to realize multiple gray scales. An area gray scale method or a time gray scale method is often used as the method for multiple gray scales.
The area gray scale method is a method for expressing a gray scale by controlling the area of a lighting portion. That is, a gray scale is expressed by dividing one pixel into a plurality of subpixels and controlling the number or the area of lighting subpixels (e.g., see Reference 1: Japanese Published Patent Application No. H11-73158 and Reference 2: Japanese Published Patent Application No. 2001-125526). In the area gray scale method, the number of the subpixels cannot be increased; therefore, it is difficult to realize high definition and multiple gray scales. This is given as a disadvantage of the area gray scale method.
The time gray scale method is a method for expressing a gray scale by controlling the length of a light emitting period or the frequency of light emission.
That is, one frame is divided into a plurality of subframes, each subframe is weighted with respect to the frequency of light emission, the light emitting period, or the like, and then the total weight (the sum of the frequency of light emission or the sum of the light emitting period) is differentiated for each gray scale, thereby expressing a gray scale. It is known that display failure called a pseudo contour (or a false contour) occurs when such a time gray scale method is used, and measures against the failure are considered (e.g., see Reference 3: Japanese Patent No. 2903984, Reference 4: Japanese Patent No. 3075335, Reference 5: Japanese Patent No. 2639311, Reference 6: Japanese Patent No. 3322809, Reference 7: Japanese Published Patent Application No. H10-307561, Reference 8: Japanese Patent No. 3585369, and Reference 9: Japanese Patent No. 3489884).
However, even though various methods for reducing a pseudo contour have been suggested, an effect of reducing a pseudo contour has not been sufficiently obtained.
For example, FIGS. 1A and 1B in Reference 4 are referred to, and it is assumed that a gray scale level of 127 is expressed in a pixel A and a gray scale level of 128 is expressed in a pixel B next to the pixel A. A lighting or non-lighting state in each subframe of this case is shown in FIGS. 80A and 80B.
Here, how to interpret FIGS. 80A and 80B is described. FIGS. 80A and 80B are diagrams showing a lighting or non-lighting state of the pixels in one frame. The horizontal direction of FIGS. 80A and 80B indicates time, and the vertical direction thereof indicates each position of the pixels. The length of the horizontal direction of a square in FIGS. 80A and 80B indicates a relative length of a lighting period in each subframe. The area of each square in FIGS. 80A and 80B indicates brightness of the pixel in each subframe.
For example, FIG. 80A shows a case where only the pixel A or the pixel B is seen without moving the eyes. A pseudo contour does not occur in this case. This is because the eyes sense the brightness in accordance with the sum of the brightness of a place where a line of sight passes. Thus, the eyes sense the gray scale level of the pixel A to be 127 (=1+2+4+8+16+32+32+32), and the eyes sense the gray scale level of the pixel B to be 128 (=32+32+32+32). That is, the eyes sense an accurate gray scale.
On the other hand, FIG. 80B shows a case where a line of sight is moved from the pixel A to the pixel B or from the pixel B to the pixel A. In this case, the gray scale level is sometimes perceived as 96 (=32+32+32), and the gray scale level is sometimes perceived as 159 (=1+2+4+8+16+32+32+32+32) depending on a movement of the line of sight. Although the gray scale level is supposed to be perceived as 127 and 128, the gray scale level is perceived to be 96 or 159, and thereby a pseudo contour occurs.
FIGS. 80A and 80B show a case of an 8-bit gray scale (256 gray scales). Next, FIG. 81 shows a case of a 6-bit gray scale (64 gray scales). In this case also, the eyes sometimes sense the gray scale level to be 16 (=16), and sometimes sense the gray scale level to be 47 (=1+2+4+8+16+16) in accordance with the eyes' movement. Although the gray scale level is supposed to be perceived as 31 and 32, the gray scale level is perceived to be 16 or 47, and thereby a pseudo contour occurs.